The thought of developing fully automatic working tools is old. Such working tools may relate to devices for cutting grass, which from now on will be called a robot. Despite this fact, such products have only in recent years reached the market. One such example is the robotic lawn mower Auto Mower™. It mows a surface by moving across the surface within the area to be worked on. Loops are used to keep the robot within a delimited area. These comprise electrical conductors transmitting signals, which are sensed by a sensing device onboard the robot and thus controlling the movements of the robot from outside. The loops are used, for instance, to mark the boundary, which the robot must not exceed and to make the robot distance itself from the loop.
The sensing device normally consists of at least a receiving device for sensing for instance magnetic fields, a control unit connected to the receiving device to process the received signal and a motor control unit connected to the control unit to control the movements of the robot based on the processed information. When the robot approaches a loop, the system senses a change in magnetic field strength. The control unit processes the information and chooses to direct the robot in a certain direction dependent on which function that is activated.
A disadvantage with the previous loop system is that the robot follows a laid down loop in order to direct the robot towards a certain place, which can be represented by a charging station for the robot's batteries and/or parking place for when the robot is not working. To direct the robot towards this place, one must necessarily put down a navigational control station comprising a big closed loop so that the robot frequently intersects it during its normal movements and thereby has the possibility to start following the loop. A further disadvantage is that present types of navigational control stations normally direct the robot along a path, which is repeated each time the robot is directed. Thereby the robot's wheels will cause wear along the surface area they pass. The present invention has been developed with the intention to remedy these disadvantages.